The Single Radial Immunodiffusion (SRID) technique was adapted for testing rabies vaccines for potency. The glycoproteins from the Pitman-Moore (PM) and Flury-LEP strain of virus were purified and a sheep antiserum was prepared against the PM glycoprotein. We received antisera against the Flury-LEP and ERA strains from NIBSC (London). The glycoprotein content of each vaccine strain tested was found to vary when tested using the different antisera and when compared to the International Reference Preparation (strain PM). However, if a homologous vaccine strain was used as the reference, the relative potency values remained constant. These data indicate that, ideally, the standardization of different strains of rabies virus vaccine requires a homologous reference for each vaccine strain. Vaccine lots containing PM, SAD and Flury-LEP strains were tested by SRID and compared to the manufacturers NIH results. These data were analyzed using correlation and regression analyses. The results showed that only the PM vaccines has a correlative relationship between the SRID and NIH tests. The results also suggest that, for the PM strain, the SRID is a less variable means of determining vaccine potency. We participated in a WHO collaborative study on the determination of Glycoprotein antigen content in human rabies vaccines.